Production of a form of alumina whiskers

Abstract

The invention relates to the production of alumina whiskers from alumina-bearing material by preparing an intimate mixture of such material and carbon and then passing a stream of gaseous aluminium trihalide and an inert carrier gas thereover in a first zone at an elevated temperature, usually 1400°C to 1800°C, at which reaction occurs to form aluminium monohalide and carbon monoxide. The gaseous stream comprising the two latter materials is then passed to a second zone maintained at a less elevated temperature than the first zone, usually 1400°-1000°C, at which temperature the reaction is reversed to produce a gaseous stream carrying alumina and carbon. Under conditions of low supersaturation the alumina deposits as single crystal alumina whiskers from which any carbon can be removed by an oxygen-containing gas at temperatures of 800°-1000°C.

What we claim is: 
     
       1. A process for the production of alumina whiskers comprising contacting an intimate mixture of particulate alumina-bearing material and a particulate form of carbon with a stream of gaseous aluminium trihalide in a first reaction zone maintained at an elevated temperature at which reaction occurs to form aluminium monohalide and and carbon monoxide, transporting the gaseous products comprising said monohalide and carbon monoxide to a second reaction zone maintained at a lower temperature than said first reaction temperature, this said lower temperature being one at which reversal of the reaction which has occurred in said first reaction zone occurs with formation of a gaseous stream supersaturated with respect to alumina under the conditions prevailing in the second reaction zone and deposition of the desired whisker form of alumina. 
     
     
       2. A process as claimed in claim 1, in which anhydrous aluminium trihalide is formed from its elements in a preliminary reaction zone by passing a mixture of the vapour of the corresponding halogen and an inert carrier gas over high purity molten aluminium. 
     
     
       3. A process as claimed in claim 2, in which the resulting reaction mixture including the aluminium trihalide formed in the preliminary reaction zone is passed over heated unmolten aluminium in a form having a large surface area relative to the volume thereof to ensure that no unreacted halogen is present in the gaseous mixture being passed to the first reaction zone. 
     
     
       4. A process as claimed in claim 1, in which said particulate form of carbon is graphite particles. 
     
     
       5. A process as claimed in claim 1, in which said intimate mixture of particulate alumina-bearing material and particulate carbon comprises substantially 3 parts by weight of alumina and 1 part by weight of carbon. 
     
     
       6. A process as claimed in claim 1, in which the temperature in said first reaction zone is between 1400° and 1800°C. 
     
     
       7. A process as claimed in claim 6, in which the temperature in said first reaction zone is between 1500° and 1700°C. 
     
     
       8. A process as claimed in claim 6, in which the temperature in the second reaction zone is 200° to 400°C lower than the temperature in the first reaction zone but is less than 1400°C. 
     
     
       9. A process as claimed in claim 1, in which a subatmospheric pressure is maintained in the first reaction zone. 
     
     
       10. A process as claimed in claim 1, in which continuous flow occurs from the first reaction zone to the second reaction zone and the pressure in the two zones is substantially the same subatmospheric pressure. 
     
     
       11. A process as claimed in claim 10, in which the pressure is less than 0.5 atmosphere absolute pressure. 
     
     
       12. A process as claimed in claim 1 in which the desired whisker form of alumina is deposited by condensation upon the surface of a substrate of α-alumina located in the second reaction zone. 
     
     
       13. A process as claimed in claim 3, which further comprises discontinuing the supply of the gaseous stream containing aluminium trihalide when alumina whiskers have been formed and replacing it by a stream of oxygen-containing gas while continuing to heat said second reaction zone to at least 800°C, the other components of said gas being inert with respect to alumina at raised temperatures to convert deposited carbon contaminating the whiskers of alumina into an oxide of carbon which oxide is then removed from the second reaction zone by passage of said oxygen-containing gas. 
     
     
       14. A process as claimed in claim 8, in which the second cooler reaction zone is a zone of diminishing temperature in the direction of flow of the reaction mixture. 
     
     
       15. A process for the production of alumina whiskers which comprises the steps of contacting an intimate mixture of particles of alumina-bearing material and particles of carbon with a gaseous stream comprising a mixture of aluminium trihalide vapour and an inert carrier gas in a first reaction zone maintained at an elevated temperature of at least 1400°C at which temperature reaction occurs to form a gaseous mixture comprising the corresponding aluminium monohalide and carbon monoxide, transporting said gaseous mixture to a second reaction zone maintained at a less elevated temperature than said first reaction zone but above 1000°C at which latter temperature the reaction which has taken place in said first reaction zone is reversed with formation of alumina and carbon so as to obtain a gaseous mixture containing a low supersaturation of alumina and deposition of alumina takes place from said mixture in the desired whisker form.